This invention relates to methods and devices for volumetric computed tomograph (CT) reconstruction and more specifically to methods and devices for Grangeat-type half-scan cone beam volumetric CT reconstruction for short and long objects in the helical and circular scanning cases.
In at least one known CT imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, generally referred to as the “imaging plane”. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In other known CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object.
One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back-projection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which can be used to control the brightness of a corresponding pixel on a cathode ray tube display.
The two-dimensional methods discussed above can reconstruct a slice of the measured object. If a volume segment needs to be reconstructed, the complete procedure can be performed slice-by-slice with a small movement of the object or of the source-detector system between each slice.
A more efficient acquisition setup for volumetric CT uses a two-dimensional detector. The rays then form a cone with its base on the detector and its apex on the source. An x-ray source naturally produces a cone of rays, so cone-beam acquisition not only increases the scanning speed, but also makes better use of the emitted rays otherwise wasted by collimation.
Modern CT scanners are rapidly moving from fan-beam towards cone-beam geometry. Current micro-CT scanners are already in cone-beam geometry. Half-scan CT algorithms are advantageous in terms of temporal resolution and are widely used in fan-beam and cone-beam geometry.
A helical source trajectory is natural for volume scanning of long objects. A continuously translated object and a rotating source-detector system yield a helical source trajectory around the object. Helical scanning has been used for many years with one-dimensional detectors and has now been extended for use with multi-row detectors with potential applications for two-dimensional detectors in the medical imaging field.
Sixteen-slice helical computerized tomography scanners are commercially available. An efficient way to acquire volumetric patient data is to use a helical source trajectory together with a multi-row detector.
With the increasing number of detector rows, the cone-beam CT systems are expected to be available in the near future. New applications made possible by these new fast volumetric imaging technologies include, but are not limited to, cardiac and lung examinations, CT angiography, and interventional procedures. In those applications, high temporal resolution is one of the most important requirements.
Half-scan techniques have been developed to improve the temporal resolution for axial and spiral CT images. Various half-scan cone-beam reconstruction methods are known in the art.
The first practical algorithm for three-dimensional reconstruction from cone beam projections acquired from a circular source trajectory was the Feldkamp method. Various Feldkamp-type algorithms were developed for half-scan cone-beam reconstructions from half-scan data collected from circular and helical loci. This method has certain limitations, most notably off-mid-plane artifacts that occur because the approximation error becomes larger as it goes away from the mid-plane.
It is therefore desirable to provide a method and apparatus for a half-scan cone-beam, three dimensional reconstruction of computed tomographic images that performs appropriate data filling using the Grangeat approach and suppresses the off-mid-plane artifacts associated with the Feldkamp-type algorithms.